Identify the formula for net force.

4) (ب) (1) Step 1: Identify the formula for net force. According to Newton's second law of motion, the net force ($F$) acting on an object is equal to the product of its mass ($m$) and acceleration ($a$). $F = ma$ Step 2: Substitute the given values into the formula. Given mass $m = 1200 \text{ kg}$ and acceleration $a = 3 m/s^2$. $F = (1200 kg) \times (3 m/s^2)$ Step 3: Calculate the net force. $F = 3600 \text{ N}$ The magnitude of the net force is $\boxed{*3600 N*}$.

4) (ب) (2) Step 1: Identify the given values and the principle of moments. For the seesaw to be balanced, the sum of clockwise moments must equal the sum of counter-clockwise moments about the pivot. Given: Distance of boy from pivot ($d_{boy}$) = 2 m Distance of man from pivot ($d_{man}$) = 1.5 m Weight of man ($W_{man}$) = 600 N Acceleration due to gravity ($g$) = 10 m/s²

Step 2: Calculate the moment due to the man. The man creates a clockwise moment. $Moment_{man} = W_{man} \times d_{man}$ $Moment_{man} = 600 N \times 1.5 \text{ m}$ $Moment_{man} = 900 N\cdot\text{m}$ Step 3: Apply the principle of moments to find the boy's weight. For equilibrium, the boy's counter-clockwise moment must equal the man's clockwise moment. $W_{boy} \times d_{boy} = Moment_{man}$ $W_{boy} \times 2 m = 900 N\cdot\text{m}$ $W_{boy} = \frac{900 N\cdotm}{2 m}$ $W_{boy} = 450 \text{ N}$ Step 4: Calculate the boy's mass. Weight is related to mass and gravity by the formula $W = mg$. $m_{boy} = \frac{W_{boy}}{g}$ $m_{boy} = \frac{450 N}{10 m/s^2}$ $m_{boy} = 45 \text{ kg}$ The mass of the boy is $\boxed{*45 kg*}$.

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